Quorum sensing, the ability of a cell to sense the density of neighboring cells, plays a vital role in eukaryotic processes ranging from organ formation and regeneration to disease progression in trypanosome infections. The social amoeba Dictyostelium discoideum is the simplest eukaryote to display quorum sensing, and due to its genetic tractability presents itself as an excellent organism in which to study this phenomenon. D. discoideum links its development to quorum sensing by using one G protein mediated signaling pathway to regulate a second G protein pathway. While G-protein signaling cross-talk is known to be critical for regulating neuronal function in mammals, this is the first example of such signaling being used in quorum sensing. In addition, the mammalian work is hindered by the lack of physiological models and the difficulties of genetics in the systems studied. Thus, the components that link one G protein signaling pathway with another remain unclear in both neurons and quorum sensing. The long-range goal of this research is to identify how organisms monitor and regulate the cell density of different tissues. The objective of this proposal is to further identify the components involved in G protein signaling cross-talk in quorum sensing in D. discoideum. The central hypothesis will be tested and the overall objective of this proposal will be accomplished by pursuing 5 specific aims. The aims are: 1) Determine whether OKC is required for quorum sensing, and if it is activated by translocation. 2) Determine the activity, cellular localization and binding partners of PldB during G protein cross-talk in quorum sensing. 3) Determine whether PLD is required for proper cellular differentiation. 4) Determine the epistatic relationship between PKC, PLD and RGS in G protein cross-talk 5) Identify other genes involved in G protein cross-talk in quorum sensing in a second site suppressor screen.